Is there a function in Project Online that replicates Master project and subprojects features?

This is still supported in Project Online (you can publish a Master project for example) BUT you may not want to use it. The reason being that Project Online is way, way more flexible when it comes to grouping projects and opening up several projects into a single "virtual" project in Microsoft Project Desktop. For example if you want to group by programme, and there is an Enterprise custom field for Programme (linked to a lookup table), you can easily configure a Project Center view to group on Programme. Then you can select all the projects that make up your selected programme and open them all at once in the Project Desktop client. You now have a virtual project based on programme name containing all the projects in that programme. You can then set up x-project links or do reporting easily from this view, including using Visual Reports. You could also save this "virtual" project as a local mpp file (since the file oly contains "links" to the actual projects in Project Online. Suppose then you wanted to view all projects grouped by Customer, or location, or cost center code, or by Stakeholder, or by Project Type (EPT), or by any other attribute you wanted to assign to a project. This approach makes grouping/filtering your projects easy and straightforward and is way more functional than the old Master/Sub project approach. bear in mind too that once a project is inserted as a sub-project into a Master Project, it can't be inserted into any other Master Project. So Project Online gives you oddles more possibilities that the old Master Project/Sub project mechanism ever did. Also, if you use the old approach and publish the Master Project in Project Online you have to be careful about whether you show sub projects in the Project Center View because any numeric or cost based fields will cause the rollup value to be duplicated or triplicated - hence the ability to turn off "sub projects" in the Project Center View , but this is easily forgotten, so we don't recommend this older approach in Project Online. 🙂